Direct sequence CDMA codes such as used in the 2G IS95 cellular system and the third generation TDD and FDD systems both in the US (CDMA2000) and Europe (UTRA) are defined to be orthogonal at the transmitter. In an ideal non-dispersive channel such as would be associated with line of sight propagation this orthogonality is preserved and the codes remain orthogonal at the receiver. A matched filter operation with replicas of the transmitter codes will separate the codes completely at the receiver and there is no loss of performance induced by the channel.
Under less favourable propagation conditions such as is commonly found in vehicular macro cells (>1 km radius) and for metropolitan or urban propagation there is significant multipath from buildings and the exact orthogonality of the codes is not preserved at the receiver. This means that there is cross talk between the codes and this is called “self noise” whereby interference (I) between code words becomes a significant degradation in addition to the usual thermal noise (N) which is always present. This self noise component means that signal to thermal noise ratios must be increased to maintain the overall carrier (C) to N+I ratio and this needs increased power to be used at the transmitters in either direction, causing increased interference to other cells and a drop in system capacity.
To prevent this loss, various measures can be taken to reduce the effects of channel dispersion. In time division duplex (TDD) third generation downlink systems linear minimum mean square error receivers are likely to be used as defined in A. Klein, G. K. Kaleh, P. W. Baier: “Zero forcing and minimum mean square error equalization for multiuser detection in code-division multiple access channels”, IEEE Trans. Veh. Tech. 45(2), May 1996, pp. 276–287. This technique however has as a very large DSP load which is undesirable. In Frequency Division Duplex (FDD) systems with long code words the use of interference cancellation or multi-user decision feedback is a promising solution as discussed in H. Holma, A Toskala, “WCDMA for UMTS”, Wiley 2000.
An alternative solution to the multipath problem is the use of orthogonal frequency division multiplexing (OFDM) in which lengthy orthogonal sinusoids are used as signalling waveforms as described in R. van Nee and R. Prasad; “OFDM for wireless multimedia communication”, Artech, 2000. The duration of these sinusoids is many times the dispersion time of the channel, The effects of multipath are avoided by transmitting a cyclic prefix before the sinusoid as shown in FIG. 1, which has the effect of allowing the multipath to settle before the start of the real signalling word. This prefix, which is longer than the multipath duration, is excised at the receiver and the remaining part of the tone, having the original duration, is applied to a discrete Fourier Transform in the usual way.
When these received tones, with prefix removed, are matched-filtered with a Fast Fourier transform there is no intersymbol interference at the output and the signals remain orthogonal. Thus there is no self noise. The effect of the multipath is transformed from a time domain problem into a frequency dependence of the channel, i.e. different tones have a different received amplitude. However, the DSP load for OFDM is quite high and battery powered handsets, in particular, are put at a disadvantage when using this technique.
A further problem with OFDM is that many tones are transmitted simultaneously at the transmitter, typically 512 to 2048 and the peak to average Power Amplifier (PA) power ratio (PAPR) is high. This causes an increased cost at the transmitter PA which already accounts for typically 50% of the electronics component of a base station cost even for binary CDMA codes.